For new sites, or site rebuilds, it's worryingly common for search engine optimisation to be the last thing considered.
From my experience, most sites that have SEO tacked on as an after-thought tend not to rank well in Google as a result.
Of course, as an SEO, I'm biased, but you can quite often spot the sites that have thought about SEO while the site was being planned, rather than after it was built. They're usually the ones with all the traffic.
If you're in a position where you're looking to build (or rebuild) a site, there are a lot of reasons to think about SEO from the start, not least because SEO is one of the most undervalued marketing channels around.
You get to choose the right CMS
There's a whole range of content management systems that have their own benefits and drawbacks.
When SEO is baked into your plans before you choose your CMS, you can reduce your chance of finding any hidden surprises, like product pages that 404 if the product is out of stock, or thousands of pages of duplicate content and an inability to 301 redirect.
These issues are surprisingly common, especially with bigger CMS providers. If you're going to sign a contract for an expensive CMS, get an SEO to talk to the vendor first, and let them play with a demo build - after all, you wouldn't buy a house without getting a surveyor in first.
You can plan a proper site structure
When you're thinking about SEO before your site is built, you can plan how best to link each page together effectively, and how to make your navigation work well for search engines, as well as for your visitors.
You can make sure that you can reach important pages in as few clicks as possible from your homepage, and you can generally build in a way that's most likely to perform best (SEO Gadget has an excellent post on site architecture if you want to find out more).
If you're just thinking about SEO after your site is live, then your site structure can be one of the hardest things to change.
You can make sure your site is accessible
Even if you have an excellent CMS, a load of great content and a lovely design, you may find that your site isn't as accessible to search engines as it could be.
You may find that there's text that the search engines can't read because it's been placed within a background image, for example. While these things are usually fixable, it's better to not have the issue in the first place. Once it's built and live, it takes longer to change.
Having an SEO on board while your site is being planned, designed and built near enough ensures that you won't see unnecessary issues like this affect you.
Your tracking won't kill you
It's much rarer these days, but you still see things like session IDs used to track visitors. There's a load of different ways in which your web analytics can be set up, some of which can significantly damage your chances of ranking well.
Session IDs, where each visitor gets a unique tracking code in the URL, is one such example that can lead to problems. In this interview with Google's Matt Cutts by Eric Enge, Eric asks Matt what he thinks of session IDs, and Matt's answer begins with "don't use them".
If your site is built with SEO in mind, then you can make sure that your tracking isn't set up in a way that can cost you valuable search traffic.
Your keyword research is an advantage, not an after-thought
When an SEO consultant is brought on after a site is built, it generally means the pages on the site have already been determined. The content has been decided, the pages are live and it's the job of the SEO to optimise them.
Typically, the keyword research then needs to fit around what's already on the site. This happens more often than it should, and it's completely backwards. The best scenario is for the keyword research to be done before the site is built, during the planning stages and, ideally, as early as possible.
This way round, you've got a huge advantage, it's easier to build pages around keywords that you know people are more likely to use, your category pages can be named better and you know how best to describe your products.
You may discover a demand for similar products that you hadn't thought about before, or a wealth of related informational keywords that could send potential customers your way. The point is, the keyword research should inform which pages are created, not the other way round.
You can set your site up to help build you links
If you have a full SEO strategy in mind before you begin work on building the site, you may find you can do some of the more interesting link building techniques to better effect.
One of the best examples of this is being able to have a blog sit on a folder on your main site, instead of tucked away on a subdomain or on a completely different domain. This way, you can be sure that you'll be making the most of all of the links the blog picks up.
There are a lot of content management systems built for e-commerce that don't allow blogs to be included on a folder, but that hasn't stopped Kiwi Collection or Sofa.com from being able to do it, most likely because they were thinking about SEO before it was built, instead of tacking it on at the end (note: neither of those sites are clients).
It may seem like a small thing, but it could prove to be hugely beneficial to them in the long run.
Whether your link building strategy involves writing amazing blog posts, having content that can be easily embedded or even using badges - if you plan it in from the start, you may find that it's easier for you to pick up links, or that you get more from them than your competitors do.
The bottom-line is, if you have SEO in mind before your site is built, you should find that you don't have the technical issues that some of your competition does, and you'll be in a better position to get on with the job of building links and getting traffic, instead of putting out fires and playing SEO catch-up.